1. Field of the invention
The present invention relates to a light wavelength converter used for information processors, such as an optical memory disc system or a laser beam printer, and optical measuring instruments where it is required to convert a wavelength of laser beams into a short wavelength zone.
2. Description of the prior art
An information processor, such as an optical memory disc system or a laser beam printer, and an optical measuring instrument use laser beams emitted from a semiconductor laser device that has a good quality in the focusing spot and directivity. In general, a laser beam emitted from a semiconductor laser device is a near infrared beam having an oscillation wavelength of 780 nm or 830 nm.
In recent years, in order to increase the amount of information to be processed in the information processors, or to enhance the measuring accuracy in the optical measuring instruments, short wavelength laser beams are required. In the information processor, the laser beam emitted from the semiconductor laser device is condensed at a predetermined place so as to write the information or images. The wavelength of the laser beam and the diameter of the focusing spot are usually proportional, so that, as the wavelength of a laser beam becomes shorter, the diameter of the focusing spot is reduced. As the diameter of the focusing spot is reduced, the amount of information (i.e., the recording density) to be written into the optical memory disc system is increased.
In the laser beam printer, the size of images to be printed can be reduced in proportion to the reduction in the wavelength of the laser beam, which means that the recording density is increased and that the resolution is increased. Moreover, if green and blue laser beams are easily obtained, a high speed and high resolution color printer can be achieved by combining a commonly used red laser beam. In optical measuring instruments, the measuring precision is enhanced by shortening the wavelength of the laser beam.
Recently, it is known that a semiconductor laser device using III-V semiconductor materials emits laser beams having oscillation wavelengths in the 600 nm level (for example, 680 nm), but so long as group III-V semiconductor materials are used, it is difficult to obtain laser beams having much shorter wavelengths. Therefore, efforts are made to develop semiconductor laser devices using ZnSe, ZnS and other group II-VI semiconductor materials, but at present even p-n junctions have not yet been realized. As is evident from this fact, no semiconductor laser devices capable of oscillating shortwave green and blue laser beams are available because of the unavailability of suitable materials. As a substitute, a large-scaled laser device such as an argon ion laser device and other gas lasers are used to obtain green, blue and other shortwave laser beams.
To solve this problem, methods for obtaining green and blue shortwave laser beams have been proposed without using large-scaled gas lasers but with the wavelength of laser beams oscillated by solid-state lasers and semiconductor laser devices. One method proposes sum frequency generation, that is, a plurality of optical frequencies are mixed to change the wavelengths of a laser beam. A typical example is the generation of second harmonics or third harmonics where two or three waves having the same frequency are mixed. Currently, by the second harmonic generating method, green laser beams with a wavelength of 0.53 .mu.m are generated using a YAG (yttrium aluminum garnet) laser with a wavelength of 1.06 .mu.m. Blue laser beams with a wavelength of 0.415-0.42 .mu.m is also generated by using a semiconductor laser with a wavelength of 0.83-0.84 .mu.m.
An example of the generation of second harmonics using semiconductor laser beams with a wavelength of 0.84 .mu.m is reported in "Oyo Buturi" (vol. 56, No. 12, pages 1637-1641 (1987)). According to the report, an optical waveguide is formed on a LiNbO.sub.3 substrate by a proton-exchange method so as to generate second harmonics with an optical output of 0.4 mW at a conversion efficiency of 1% by using semiconductor laser beams having a wavelength of 0.84 .mu.m and an optical output of 40 mW. When the semiconductor laser beams are introduced into the optical waveguide, which is 2.0 .mu.m wide and 0.4 .mu.m deep, second harmonics are emanated into the substrate at an incline of approximately 16.2.degree. thereto. At this point, the second harmonics and the fundamental waves are automatically phase-matched, thereby providing no restriction on the angle between the beam and the crystal and the temperature of the crystal. However, the output of the second harmonics, 0.4 mW, is too small to be utilized, for example, for an optical memory disc system which requires at least ten times this amount of output.
Since the wavelength conversion efficiency increases in proportion to the density of fundamental waves, the output of harmonics is proportional to the second power of the density of the fundamental waves. "IEEE Journal of Quantum Electronics" (vol. 24, No. 6, pages 913-919 (1988)) discloses a method of using an optical resonator in order to increase the density of fundamental waves. In the light wavelength converter proposed in this literature, as shown in FIG. 8, fundamental waves emanated from a YAG laser 91 are introduced into an MgO doped LiNbO.sub.3 crystal 92 from one of the end faces. The crystal 92 is a non-linear optical crystal with the opposite end faces being precisely finished and coated with a reflecting film. The crystal 92 is placed in an oven 96, and heated to a predetermined temperature. The fundamental waves introduced into the LiNbO.sub.3 crystal 92 reflect on the end and side faces thereof and circulate along the same optical paths with the crystal 92. Because of the reduced optical loss in the optical path of the fundamental waves, if the optical resonance conditions are satisfied, the intensity of the fundamental waves circulating along the optical paths amounts to be about ten times that of the incident light beams. The portion of the harmonics that is phase-matched with the fundamental waves is emitted from the end face of the crystal 92 opposite to one from which the fundamental waves are introduced, which is indicated by the broken lines in FIG. 8. The crystal 92 is sandwiched between a pair of electrodes 93a and 93b. A voltage is applied to the crystal 92 through the electrodes 93a and 93b, thereby controlling the optical resonance inside the crystal 92. The voltage applied between the electrodes 93a and 93b is controlled by the reflection of the fundamental waves on the end face of the crystal 92 when it receives the fundamental waves. More specifically, the reflected light received by a detecting device 94 is converted to a predetermined electric signal that is transmitted to a control circuit 95, where the voltage applied between the electrodes 93a and 93b is controlled so as to minimize the reflected light. In this way, with the use of the wavelength converter mentioned above, by inputting 52.6 mW YAG laser beams to the crystal 92, high harmonics having a large output of 29.7 mW is obtained with the conversion efficiency of 56%.
To put the above-mentioned light wavelength converter into practical use, there are problems:
First, both end faces of the non-linear optical crystal 92 must be carefully ground so as to achieve a desired delicate curvature and, then, be covered with a dielectric reflection film, which involves a difficult process. Second, it is difficult to satisfy the phase-matching conditions between the fundamental waves and the harmonics because of the susceptibility thereof to any disaccord in position between the incident light and the non-linear optical crystal. To achieve this, highly precise positioning between light and the non-linear optical crystal is required. Furthermore, in order to satisfy the phase-matching conditions, the non-linear optical crystal 92 must be strictly controlled to maintain a predetermined temperature (e.g., 107.degree. C.), which, for example, requires that the crystal 92 be heated in the oven 96.
A practical light wavelength converter designed to overcome the above-discussed disadvantages uses a waveguide type optical resonator. The Inventors have proposed a light wavelength converter, where a waveguide functions as a loop-shaped optical resonator, impinging harmonics upon a substrate, which is disclosed in Japanese Patent Application No. 1-77823 (corresponding to copending commonly assigned U.S. patent application Ser. No. 07/498,573 filed Mar. 24, 1990 (now U.S. Pat. No. 5,046,802), naming Yamamoto et al as inventors). As shown in FIG. 9, this light wavelength converter comprises a loop-shaped optical waveguide 82 formed on a crystalline substrate 81 (e.g., Y-cut MgO doped LiNbO.sub.3) producing non-linear optical effects. Laser beams (fundamental waves) generated from a laser beam source 84 are introduced to the optical waveguide 82 through an optical system 85 and are circulated in the optical waveguide 82. While the fundamental waves spread within the linear harmonics generating part 82a, they are converted into second harmonics A, which are emitted into the substrate 81 and output from an end face thereof. The fundamental waves spreading within a monitor wave generating part 82d, which is located opposite to the linear harmonics generating part 82a and roughly in parallel thereto, are converted into second harmonics B, which are emanated toward a detector 86 from the end face of the substrate 81 into which the fundamental waves are introduced. The detector 86 converts the received second harmonics B into a predetermined electric signal and sends it to a control circuit 87. The control circuit 87 controls a voltage applied to a pair of electrodes 83 and 83 placed on each side of an initial portion of the monitor wave generating part 82d in the optical waveguide 82, thereby changing the light wavelength of the fundamental waves spreading within the monitor wave generating part 82d, thus obtaining a maximum output of the second harmonics received from the detector 86 so as to satisfy the resonance conditions in the light waveguide 82. The fundamental waves which have passed through the monitor wave generating part 82d are returned to the harmonics generating part 82a through an non-symmetrical linked part 82f.
In the light wavelength converter of the previous invention, the fundamental waves are likely to cause a coupling loss of approximately 1.0 dB (20%) when they are returned to the harmonics generating part 82a through the non-symmetrical linked part 82f. If this loss is combined with a spreading loss of 10% or more occurring in other parts of the loop-shaped light waveguide 82, the three or more times amplification of the fundamental wave will be impossible.
Another disadvantage of the previous invention is that the fundamental waves converted into the second harmonics in the phase-adjusting method according to Cerenkov radiation, in which harmonics are generated from the whole area of the optical waveguide. Consequently, the shape of the irradiated beams of the harmonics are axially non-symmetrical. In this way the usable harmonics are limited to those generated in one direction from the optical waveguide in spite of being generated from the whole area of the optical waveguide with the remaining harmonics left unused, thereby reducing the utilization of the fundamental waves. In addition, the irradiation beams of the harmonics cannot be focused to the diffraction limit because they have no axially symmetrical shape, thereby limiting the range of application.